Conventional dielectric ceramic filters offer high performance with scalability which make them ideally suited for use in mobile and portable radio transceivers. They are usually comprised of a plurality of dielectric ceramic resonators that are typically foreshortened, short-circuited quarter-wavelength coaxial.
In FIG. 1, there is illustrated a prior art dielectrically loaded bandpass filter 100 employing a conventional input connector 101 and a conventional output connector 103. Such a filter is more fully descirbed in U.S. Pat. No. 4,431,977, entitled "Ceramic Bandpass Filter" and is incorporated by reference herein. The filter 100 comprises a block 105 which is generally made of a dielectric ceramic material with a conductive material selectively plated thereon, having a low loss, a high dielectric constant, and a low temperature coefficient of the dielectric constant, e.g., a ceramic compound comprising barium oxide, titanium oxide and zirconium oxide.
A dielectric filter such as that of the block 105 of the filter 100 is generally covered or plated, except for areas 107, with an electrically conductive material, for example, silver or copper. The dielectric filter such as the block 105 includes a multiplicity of holes 109, wherein each of the holes extends from the top surface to the bottom surface thereof and is likewise plated with the electrically conductive material. The plating of the holes is electrically connected with the conductive plating covering the block 105 at one end side of the holes 109 and is isolated from the plating covering the block 105 at the opposite end side of the holes 109. Further, the plating of the holes 109 at the isolated one end side may extend onto the top surface of the block 105. Thus, each of the plated holes 109 is essentially a foreshortened coaxial resonator comprised of a short coaxial transmission line having a length selected for desired filter response characteristics. Although the block 105 is shown in FIG. 1 with six plated holes, any number of plated holes may be utilized depending upon the filter response characteristics desired.
The plating of the holes 109 in the filter block 105 is illustrated more clearly in a cross sectional view cut through any one of the holes 109. As shown in FIG. 2, the conductive plating 204 on the dielectric material 202 extends through the hole 201 to the top surface with the exception of a circular portion 240 around the hole 201. Other conductive plating arrangements may also be utilized. In FIG. 3, the conductive plating 304 on the dielectric material 302 extends through the hole 301 to the bottom surface with the exception of the portion 340. The plating arrangement in FIG. 3 is substantially identical to that in FIG. 2, the difference being that the unplated portion 340 is on the bottom surface instead of on the top surface. In FIG. 4, the conductive plating 404 on the dielectric material 402 extends partially through the hole 401 leaving a portion of the hole 401 unplated. The plating arrangement in FIG. 4 can also be reversed as in FIG. 3 so that the unplated portion 440 is on the bottom surface.
Coupling between the plated hole resonators is accomplished through the dielectric material and may be adjusted or controlled by varying the width of the dielectric material and the distance between adjacent coaxial resonators. The width of the dielectric material between adjacent holes 109 (see FIG. 1) can be adjusted in any suitable regular or irregular manner, e.g., by using slots, cylindrical holes, square or retactangular holes, or irregularly shaped holes.
As shown in FIG. 1, RF signals are capacitively coupled to and from the dielectric filter 100 by means of input and output electrodes, 111, 113, respectively, which in turn, are coupled to input and output connectors 101, 103, respectively.
The resonant frequency of the coaxial resonators provided by the plated holes 109 is determined primarily by the depth of each hole, the thickness of the dielectric block, and the amount of plating removed from the top of the filter near the hole. Tuning of the filter 100 may be accomplished by the removal of additional ground plating or resonator plating extending upon the top of each plated hole. The removal of plating for tuning the filter can easily be automated, and can be accomplished by means of a laser, sandblast trimmer, or other suitable trimming devices while monitoring the return loss angle of the filter.